While not necessarily limited thereto, the present invention is adapted for use in field-effect liquid crystal light shutter displays such as those shown in U.S. Pat. No. 3,731,986, issued May 8, 1973. Light shutters of this type comprise a layer of nematic liquid crystal material sandwiched between a pair of parallel transparent plates, the plates being coated on selected areas thereof with transparent conducting material to form an alpha-numeric display. The surfaces of the plates in contact with the liquid crystal material are rubbed or otherwise aligned at right angles to each other to effect a twisted-nematic structure. Application of an electrical potential between the transparent conducting coatings will cause the nematic structure to rotate or untwist. By providing polarizers on opposite sides of the display, polarized light can be made to pass through the structure or be blocked, depending upon whether the polarizers are crossed or parallel to each other.
When a liquid crystal display of the type described above is viewed along the axis of the display (i.e., at right angles to the aforesaid transparent plates), the indicia formed by the alpha-numeric display can be readily viewed as the display is rotated through 360.degree.. Since, however, the liquid crystals are generally birefringent, the off-axis performance of the device is not uniform as the display is rotated through 360.degree.. Assume, for example, that the device is being viewed at an angle of 45.degree. with respect to the axis of the display. As it is now rotated through 360.degree., it will be seen that the off-axis directions which are parallel and perpendicular to the axis of the front polarizer on the display have good viewing. However, in-between these positions, and particularly when the display is rotated through 45.degree. from a position where the viewing angle is perpendicular or parallel to the axis of the polarizer, the image becomes more or less blurred and is exceedingly difficult to view.
It has been determined that this effect is caused by the birefringence of the liquid crystal material which is oriented normal to the surface of the aforesaid transparent plates. This effect can be very disturbing in displays with reasonable thickness; and although the effect can be reduced by making thin displays, such thin displays have a tendency to show dispersion due to differences in the index of refraction at different wavelengths, thus causing disturbing colors when the display is not activated.
Another disadvantage of present-day liquid crystal light shutters is their relatively slow response times which limits their use as television display panels and the like. For example, turn-off times of 250 to 300 milliseconds are common in liquid crystal light shutters of the twisted nematic type. Various techniques have been employed to improve the response time of liquid crystal light shutters of this type by the addition of a small amount of twisted cholesteric liquid crystal material to the nematic liquid crystals. Such efforts, however, have not increased the response time above the millisecond range. Moreover, the relatively slow response time of liquid crystal displays limits their use with time-shared multiplexing driving circuitry.